S-Curve Jerk Algorithm and Attributes



The move type, MPIMotionTypeS_CURVE_JERK supports a jerk-specified S-Curve accel/decel profile. It is only available with custom firmware. To use this feature, you will need firmware with the Alternate Trajectory Creation Algorithm option enabled. In the custom firmware, the Trapezoidal/S-Curve algorithm is replaced with the S-Curve Jerk algorithm. In the standard firmware, the S-Curve Jerk motion type is not supported and if specified, the MPI will return a "profile not supported" error. The S-Curve Jerk algorithm is ideal for making final adjustments to a move during the deceleration portion of the move and for making smoother transitions from one motion to the next. Also, the S-Curve Jerk algorithm makes it possible to exactly specify the maximum accel/decel and jerk values.


S-Curve Algorithm
: Notice the mpiMotionModify calculations assume the initial acceleration
value is zero, which causes an abrupt change in the acceleration profile.

S-Curve Jerk Algorithm: Notice that the mpiMotionModify calculations use the initial acceleration value, which causes a much smoother transition in the acceleration profile.


Parameters for jerk values should range from a minimum of amax* amax / vmax (amax is just reached when accelerating from 0 to vmax) and a maximum of amax / sample period (amax is reached in one sample period). In the S-Curve Jerk algorithm, changes to the jerk will also change the time needed to complete a motion. For example, a large value of jerk will have a shorter time, but increase the "jerkiness" of the motion (see fig 1). Conversely, a small value of jerk will have a longer time, but a much smoother motion (see fig 2).


The S-Curve Jerk parameters, accelerationJerk and decelerationJerk are located in the MPITrajectory{.} structure. When these are non-zero, the acceleration profile uses the specified jerk and acceleration to ramp an axis(es) to constant velocity and then decelerate to a stop. If accelerationJerk or decelerationJerk is zero, the MPI will return a "parameter invalid" error.

For the move type, MPIMotionTypeS_CURVE, the MPI calculates an appropriate jerk value based on the specified velocity, acceleration, and jerkPercent. The jerk value is computed according to the following formula:

If jerkPercent is zero, the jerk value is computed so that the maximum acceleration is reached in one sample period. With the S-Curve algorithm, the time for a move would not change as jerkPercent value was varied. This is also true for this S-Curve algorithm, as long as the move reaches maximum velocity. In short moves, where maximum velocity is not reached, setting jerkPercent to be small will result in a quicker move than if you were to set jerkPercent to be large.

WARNING! The same jerkPercent values may cause different profiles with the special S-Curve Jerk algorithm than with the standard S-Curve algorithm.

Custom Firmware Limitations (Option 2 & Option 21)
Due to controller memory limitations, the standard firmware cannot fit the S-Curve Jerk algorithm. Thus, custom firmware (Option 2 & Option 21) is required. To accomodate the S-Curve Jerk algorithm, the following features are NOT available with S-Curve Jerk firmware (Option 2 & Option 21):

  • Pre-Filters· Compensation Tables
  • Sinusoidal Commutation
  • Stepper Motors
  • Camming
  • Back-up on path
  • Metrics

Also, there are some limitations with the implementation for the S-Curve Jerk algorithm:

  • Multi-axis mpiMotionModify(.) must use the SYNC_START attribute (and not use SYNC_END).
  • The velocity, acceleration, deceleration, accelerationJerk, decelerationJerk, or jerk Percent must be the same for motion modify calls as for the original motion start.
  • The ratio of distances traveled for multi-axis moves should not exceed 50,000.
  • Multi-axis mpiMotionModify(.) should not be called within the first 2 samples of a mpiMotionStart(.).



The new S_Curve algorithm behaves similarly to the previous algorithm, except for its attributes.

MPIMotionAttrMaskDELAY can now be used with any start motion, but never with motion modify.

MPIMotionAttrMaskAPPEND can be used with any motion, as long as it is not preceded by a motion that has a final velocity.

MEIMotionAttrMaskNO_REVERSAL returns a MPIMotionMessagePROFILE_ERROR if the given specifications would result in a move with a reversal, thereby preventing the move from being executed.



MPIMotionAttrMaskRELATIVE, when used with MPIMotionTypeTRAPEZOIDAL, MPIMotionTypeS_CURVE and MPIMotionTypeS_CURVE_JERK means that the final position is relative to the beginning position of the motion.

MEIMotionAttrMaskFINAL_VEL can be used with MPIMotionTypeTRAPEZOIDAL, MPIMotionTypeS_CURVE and MPIMotionTypeS_CURVE_JERK, but should be used with caution as it may not be possible for the controller to compute a trajectory to meet these criteria, which would cause a MPIMotionMessagePROFILE_ERROR to be returned, and the move to be ignored.


Multi-Axis Motion

Neither MPIMotionAttrMaskSYNC_START nor MPIMotionAttrMaskSYNC_END
If neither MPIMotionAttrMaskSYNC_START nor MPIMotionAttrMaskSYNC_END are specified, a single MPITrajectory{...} may be specified for the resultant motion of multiple axes on one motion supervisor. The motion of each axis will be synchronized with the others on the motion supervisor. The maximum velocity, acceleration, deceleration, and jerk values of the first MPITrajectory structure will be used for the global vector parameters. It will ignore any other values supplied. This cannot be used with MEIMotionAttrMaskFINAL_VEL.

MPIMotionAttrMaskSYNC_START or MPIMotionAttrMaskSYNC_END but not both
If MPIMotionAttrMaskSYNC_START or MPIMotionAttrMaskSYNC_END (but not both) is specified, each axis will move as fast as possible and either start together, or stop together. If motion is point-to-point and more than one axis on the motion supervisor has a final velocity, MPIMotionAttrMaskSYNC_START or MPIMotionAttrMaskSYNC_END must be used. MPIMotionAttrMaskSYNC_END cannot be used with motion modify.

Both MPIMotionAttrMaskSYNC_START and MPIMotionAttrMaskSYNC_END
With MPIMotionAttrMaskSYNC_START and MPIMotionAttrMaskSYNC_END, the motion for each axis will be scaled so that the motion of all axes will end at approximately the same time. The time for this motion is based on the time for the longest motion, so that the limits are not exceeded. The axes will be scaled to start and stop together, but the scaling may not be exact. Both MPIMotionAttrMaskSYNC_START and MPIMotionAttrMaskSYNC_END cannot be used together with MEIMotionAttrMaskFINAL_VEL.



MPIMotionTypeVELOCITY moves allow a final velocity to be specified without a final point.

MPIMotionAttrMaskSYNC_START and/or MPIMotionAttrMaskSYNC_END
Neither is supported for this motion type. MPIMotionAttrMaskSYNC_END cannot be used with motion modify.

MEIMotionAttrMaskFINAL_VEL is not supported for this motion type.

MPIMotionAttrMaskRELATIVE, when used with MPIMotionTypeVELOCITY or MPIMotionTypeVELOCITY_JERK, means that the final velocity is relative to the velocity at the start of the motion.

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